Stage 0 is before the cancer becomes invasive
Then, there's also a stage 0. Stage 0 is also known as pre-cancer. It's the earliest stage of breast cancer and usually begins as an abnormal growth of cells originating from the milk-producing ducts of the breast. This is otherwise known as ductal carcinoma in situ or by its acronym DCIS.
Stage 0 cancer still has the potential to become worse, meaning prompt treatment is essential.
At stage 0, cancer has not invaded any of the healthy tissue quite yet. However, it has the potential to become cancer in the future, thus warranting prompt treatment. Treatment is usually with surgical removal, combined with radiation in some cases. Usually, chemotherapy isn't used for this very early stage of breast cancer. Those who have this can still benefit from hormone therapy to reduce their risk of getting more worrisome breast tumors in the future.